This invention relates to resin formulations, and in particular to curable thermosetting resin formulations such as epoxy resins. In particular embodiments, the invention relates to resin formulations useful for making prepregs and electrical laminates.
It is known to make prepregs, electrical laminates and other composites from a fibrous substrate and an epoxy-containing matrix resin. Such processes usually contain the following steps:
(1) an epoxy-containing formulation is applied to a substrate by rolling, dipping, spraying, other known techniques and/or combinations thereof. The substrate is typically a woven or nonwoven fiber mat containing, for instance, glass fibers or paper. PA1 (2) The impregnated substrate is "B-staged" by heating at a temperature sufficient to draw off solvent in the epoxy formulation and optionally to partially cure the epoxy formulation, so that the impregnated substrate can be handled easily. The "B-staging" step is usually carried out at a temperature between 90.degree. C. and 210.degree. C. and for a time between 1 minute and 15 minutes. The impregnated substrate that results from B-staging is called a prepreg. The temperature is most commonly 100.degree. C. for composites and 130.degree. C. to 180.degree. C. for electrical laminates. PA1 (3) One or more sheets of prepreg are stacked in alternating layers with one or more sheets of a conductive material, such as copper foil, if an electrical laminate is desired. PA1 (4) The laid-up sheets are pressed at high temperature and pressure for a time sufficient to cure the resin and form a laminate. The temperature of lamination is usually between 100.degree. C. and 230.degree. C., and is most often between 165.degree. C. and 190.degree. C. The lamination step may also be carried out in two or more stages, such as a first stage between 100.degree. C. and 150.degree. C. and a second stage at between 165.degree. C. and 190.degree. C. The pressure is usually between 50 N/cm.sup.2 and 500 N/cm.sup.2. The lamination step is usually carried on for 10 to 500 minutes, and most often for 45 to 300 minutes. The lamination step may optionally be carried out at higher temperatures for shorter times (such as in continuous lamination processes) or for longer times at lower temperatures (such as in low energy press processes). PA1 (5) Optionally, the resulting copper-clad laminate may be post-treated by heating for a time at high temperature and ambient pressure. The temperatures of post-treatment are usually between 120.degree. C. and 250.degree. C. The post-treatment time usually is between 30 minutes and 12 hours. PA1 (1) an epoxy resin; PA1 (2) a curing agent, for example a polyamine such as dicyandiamide, a polyanhydrides such as a styrene/maleic anhydride copolymer, or a polyphenol or a mixture of two or more curing agents; PA1 (3) a catalyst to promote the reaction of the resin and the curing agent, such as 2-methylimidazole, 2-ethyl, 4-methylimidazole, 2-phenylimidazole, or a mixture of two or more catalysts; and PA1 (4) optionally, from 0 to 50 weight percent of a volatile organic solvent such as a ketone, a glycol ether, dimethylformamide, xylene or a mixture of two or more organic solvents. PA1 a) an optionally substituted polymer of a monovinylidene aromatic monomer, optionally having one or more further unsaturated monomers copolymerized therewith; PA1 b) an optionally substituted polyphenylene oxide; or PA1 c) an oxazolidone ring-containing compound. PA1 a) a polyisocyanate having an isocyanate functionality of from about 1.9 to about 2.1; PA1 b) a polyepoxide having an epoxide functionality of from about 1.9 to about 2.1, and optionally; and PA1 c) a chain extender, PA1 a) an optionally substituted polymer of a monovinylidene aromatic monomer and optionally one or more further unsaturated monomers copolymerized therewith; PA1 b) an optionally substitute polyphenylene oxide; or PA1 c) an oxazolidone ring-containing compound. PA1 a) an optionally substituted polymer of a monovinylidene aromatic monomer and optionally one or more further unsaturated monomers copolymerized therewith; PA1 b) an optionally substituted polyphenylene oxide; or PA1 c) an oxazolidone ring-containing compound. PA1 a) from about 20 to about 43 weight percent, based on the polyepoxide and polyisocyanate reactants, of a polyisocyanate having an isocyanate functionality of from about 1.9 to about 2.1; PA1 b) from about 80 to about 57 weight percent, based on the polyepoxide and polyisocyanate reactants, of a polyepoxide having an epoxide functionality of from about 1.9 to about 2.1; and optionally, PA1 c) a chain extender.
Electrical laminates and processes by which they are made, are described in greater detail in numerous references, such as U.S. Pat. No. 5,314,720 (May 24, 1994) and Delmonte, Hoggatt & May; "Fiber-reinforced Epoxy Composites," Epoxy Resins, Chemistry and Technology (2d Ed.) at 889-921 (Marcel Dekker, Inc. 1988).
The formulations that are used in such processes typically contain:
Viscosity is critical in laminate making processes. See, for example, Delmonte, Hoggatt & May at 903. High viscosity resins distort the position of fibers in the substrate, and are difficult to impregnate into the substrate. However, if the viscosity of the resin is too low, the resin tends to flow out of the prepreg stack during the lamination process, resulting in a laminate which is deficient in resin, such that it is very difficult to obtain a homogeneous thickness over the whole laminate. The ability to control the thickness of laminates is important in some applications. For example, recently, a new chip memory module technology has been developed to achieve faster bus/substrate speed (&gt;300 MHz) to better utilize the capabilities of faster CPUs. The critical need in this application is for extremely tight control of the dielectric constant which translates into much better thickness control of the laminate layers.
Formulations which contain liquid epoxy resins and a chain extender have not commonly been used in laminating processes, because their viscosity in the treater and prepreg is often too low. The formulations run and drip in the treater before the B-stage is complete. Furthermore, the formulations flow too much after the prepreg is put into the laminating press. The resin is forced out of the laminate and into the press, and the resulting laminate is too thin.
Extra catalysts may be added to the formulation to encourage quick reaction of epoxy resin and chain extender in the treater, so that higher molecular weight advanced resins are produced before dripping occurs. However, those catalysts also accelerate curing of the resin with the curing agent. It is difficult to prevent the viscosity from building too high for effective lamination. Moreover, formulations which contain too much catalyst have a short shelf- or pot-life, and the resulting prepregs have a short shelf-life.
It is known to incorporate thermoplastic resins in electrical laminate formulations to reduce the resin flow out (waste) by increasing the melt viscosity of the B-staged materials of the prepregs during the lamination process. A high molecular weight polymer obtained by an advancement reaction using bisphenol-A and the diglycidylether of bisphenol-A and is sold commercially by Phenoxy Associates (USA), under the trademark PKHH. This material is frequently used to reduce resin flow-out by increasing the melt viscosity of the B-staged materials without shortening the gel time. However, when PKHH is used, the Tg of the resulting laminate is adversely affected. In the case of acid anhydride cured systems (for example as disclosed in PCT/US98/01041), the secondary hydroxyl groups present in PKHH react easily at room temperature with the acid anhydride to generate acid groups in the presence of amine catalysts. These acid groups react with the epoxy groups and the pot-life/gel time of the formulations are considerably reduced and is not desirable.
There are three end products (conventionally referred to as polyoxazolidones) which can be obtained in the condensation reaction of polyisocyanates with polyfunctional epoxides, namely isocyanate-terminated polyoxazolidones, linear polyoxazolidones, and epoxy-terminated polyoxazolidones. These three possible end products and various methods for their production, are described in U.S. Pat. No. A 5,112,932 and in the references referred to therein, all of which are incorporated herein by reference. Epoxy terminated polyoxazolidones are prepared by reacting an epoxy resin with a polyisocyanate compound using stoichiometric excess of epoxy resin (isocyanate/epoxide ratio lower than 1).
U.S. Pat. No. A 4,070,416 (Hitachi) describes a process for producing thermosetting resins by mixing one equivalent or more of polyfunctional isocyanate per one equivalent of a polyfunctional epoxide in the presence of a tertiary amine, morpholine derivatives or imidazole as catalysts. The catalyst is used within a range of 0.1 to 2 weight percent, based on the combined weight of the reactants. The reaction temperature of 130.degree. C. or lower is said to result in the formation of mainly isocyanurate rings, whereas it is assumed that oxazolidone rings should be mainly formed at temperature above 130.degree. C. The resins produced are said to exhibit excellent electrical and mechanical properties and high thermal stability. They are said to have various applications as heat resistance insulation varnishes, casting resins, impregnation resins, molding resins for electrical parts, adhesives, resins for laminating boards, resins for printed circuits etc.
EP A 0,113,575 discloses powder coating compositions comprising epoxy terminated polyoxazolidone resins prepared by reacting a diepoxide and a diisocyanate in amounts which provide a ratio of epoxide equivalents to isocyanate equivalents of from 1.1:1 to 10:1 and curing agents. The polyoxazolidone resins are said to have comparatively high glass transition temperatures and provide coatings of improved resistance to cathodic disbandment. The coating compositions are applied by fluidized bed sintering or electrostatic spray methods.
Self thermosetting compositions of polyisocyanates and polyepoxides are described in U.S. Pat. No. A 4,564,651 (Markert) and U.S. Pat. No. A 4,631,306 (Markert) which disclose a method for the preparation of reaction resin molded materials and molded materials for insulating components, respectively containing oxazolidone and isocyanurate rings by mixing a polyepoxide and a polyisocyanate to from a resin mixture having a viscosity up to 7000 mpa.multidot.s at 25.degree. C. and the mole ratio of epoxy to isocyanate groups of 1:1 to 5:1; reacting the resin mixture in the presence of an imidazole or tertiary amine catalyst at elevated gelling temperature of from 80.degree. C. to 130.degree. C. to form a cross linked polymer; and heating the cross linked polymer to 130.degree. C. to 200.degree. C. to cause post hardening and produce a molded material. The molded materials are reported to exhibit improved mechanical properties.
U.S. Pat. No. A 3,334,110 (Schramm) discloses a method for preparing epoxy terminated polyoxazolidones by reacting a polyisocyanate with a polyepoxide in the presence of a catalyst mixture comprising an alcohol and tertiary amine or a quaternary ammonium salt. The epoxy terminated polyoxazolidones can be cured with epoxy curing catalysts or reacted with epoxy hardeners to give a variety of products useful in the fields of coatings, laminating, bonding, molding, foams, etc.
U.S. Pat. No. A 4,066,628 (Ashida et. al.) discloses a process for preparing polyoxazolidones by reacting an organic isocyanate with an epoxide in the presence of dialkyl zinc, zinc carboxylate, organozinc chelate compound or trialkyl aluminum as the catalyst. Polyoxazolidones prepared by this process are useful starting materials for the manufacture of a wide variety of products including foams, coatings, adhesives, elastomers and the like.
Although numerous processes for the preparation of polyoxazolidones are described in the literature, there is no disclosure nor suggestion in the known art that epoxy terminated polyoxazolidones would be useful as viscosity-modifying additives, to be added in relatively small amounts to other resin compositions, in particular to heat curable thermosetting resins such as epoxy resins, in order to improve their viscosity properties, whilst maintaining an adequate Tg in the final cured resin.
EP B 0,695,316 and U.S. Pat. No. A 5,449,737 (CIBA) also disclose the preparation of oxazolidone-containing base resins.
Although both of these CIBA references refer in general terms to materials having an epoxy equivalent weight (EEW) of from 200 to 10,000, they are concerned with "base resins", intended to be cured with a crosslinking agent, and for such purposes, a material having a low EEW (for example no more than 500) would invariably be used. If a material with a high equivalent weight (in particular, higher than 500) is used as a base resin in the production of laminates, the resin fails to wet out the glass fibers, and is therefore unsatisfactory. All of the Examples in both of the CIBA patents use an amount of isocyanate of no more than 18.7% (TDI) for the preparation of the base resin, and result in materials having an EEW of no more than 500. In practice therefore, the skilled addressee following the teaching of these references would not prepare a material having a molecular weight of 5000 or more.